1. Field of the Invention
The present invention relates to a light emitting device, in more particular, to a light emitting device, in which a photoelectric conversion efficiency is improved by improving a light extract efficiency.
2. Related Art
As one of conventional light emitting diodes, for instance, there is an LED in which a light emitting part of an active layer is not located right under an electrode provided at a light extracting side.
This type of LED comprises a GaAs substrate, a semiconductor multilayer structure formed on one surface of the GaAs substrate, the semiconductor multilayer structure comprising an n-type cladding layer, a p-type cladding layer, and an active layer provided between the n-type cladding layer and the p-type cladding layer, a circular upper electrode having a predetermined outer diameter that is formed on one surface of the semiconductor multilayer structure, and a lower electrode formed on another surface of the GaAs substrate, in which a concentric ring-shaped current injection region having an inner diameter and an outer diameter that are greater than an outer diameter of the upper electrode is formed at an interface between the GaAs substrate and the semiconductor multilayer structure, and a current confining layer is formed inside and outside of the current injection region. For example, Japanese Patent Publication No. 6-82862 (JP-B-6-82862) discloses this type of LED.
In this type of LED, the current injected from the upper electrode is flown through the concentric ring-shaped current injection region to the lower electrode, without passing through a part of the active layer located right under the upper electrode by means of the current confining layer. As a result, a light emission at the part of the active layer located right under the upper electrode is suppressed, and a main light emission is generated at another part of the active layer, through which the upper electrode is connected to the concentric ring-shaped current injection region, thereby improving the light extract efficiency.
Further, as another one of the conventional light emitting diodes, for instance, there is an LED, in which a light emitting part of an active layer is not located right under an electrode provided at a light extracting side, in order to relax a local current convergence to the active layer, as well as to control a light absorption in a semiconductor substrate. In this type of LED, a current confining layer is formed in a region right under an upper electrode at an interface between the semiconductor substrate and a semiconductor multilayer structure, a plurality of interface electrodes are arranged outside of the current confining layer in a matrix shape, the interface electrodes are isolated from each other by the current confining layer, and a reflecting layer is formed between the interface electrodes and current confining layer and the semiconductor substrate. For example, U.S. Pat. No. 6,784,462 discloses this type of LED.
In this type of LED, the current injected from the upper electrode is flown into the lower electrode via a plurality of the electrodes disposed in the matrix shape, so that it is possible to relax the local current convergence to the active layer and to reflect the light emitted from the active layer to the semiconductor substrate side at the reflecting layer toward the upper electrode side. According to this structure, it is possible to suppress the absorption of the light emitted from the active layer by the semiconductor substrate.
However, in the LED disclosed by JP-B-6-82862, since a part of the light emitted from the active layer is absorbed by the upper electrode and the GaAs substrate, there is a limit for the improvement in the light extract efficiency. In addition, in the LED disclosed by U.S. Pat. No. 6,784,462, since there is a difference in distances from the upper electrode to the respective interface electrodes in addition to the absorption of the light by the upper electrode as described above, a difference is caused in electric resistance of a plurality of current paths between the upper electrode and the respective interface electrodes through the active layer. Therefore, a difference is caused in the current flowing through the active layer, so that a difference is caused in heat generation (elevation of a temperature), luminance, and the like. As a result, there is a disadvantage in that the driving voltage and the lifetime of the LED vary among the respective devices.